I have an early 2008 MacBook Pro with the 8600m gt. I can tell you that it cannot even remotely compare to my Xbox 360 (let alone a PS3). There are games that run decently on it and there are games that run horribly. You can play most modern PC games like TF2, COD 4, Left4Dead, etc but often you have to sacrifice graphics settings, resolution, etc just to get it playable. And playability to me, is at least around 20-30 fps. Would you be happy playing a shooter on 25 fps with mediocre graphics?
I know the new MBPs have the 9600m gt, but that card is barely an improvement over mines. Maybe it'll be ok (at best) for games now, but what about the games coming out next year or in two years? On the other hand, the PS3 or Xbox 360 is guaranteed to play every single game that will ever be released for them as good as the games that were released for them since their launch day.
If you absolutely want a PC gaming rig, you can probably build/buy one for well under a 1000. It does churn slightly better graphics than the consoles but if you look up PC vs Xbox 360 vs PS3 comparison game shots, the differences are always minimal with the PC's graphics edging slightly. Keep in mind you probably have to update the gfx card every year or two to keep running new games on high.
So is the MBP a good gaming machine? No. (There are better/cheaper alternatives) Is it a decent one? Maybe. If you're fine with the graphical limitations: med settings, med res, mediocre fps... sure. Maybe you're on the go, or at a friend's house and you wanna LAN party Left4Dead, then go for it. (I do that.)
But even using it with this purpose, keep in mind that there are more powerful laptops designed from the ground up to be gaming machines, like Alienware, which are priced the same if not cheaper than MBPs. But gaming is not the focus of a MBP anyway. It's OS X. It's Final Cut Pro. It's Photoshop. It's ease of use. It's built quality. It's aesthetics. etc... But if you want to game on the side, it sure as hell can do that too, but it shouldn't be the main reason why you bought it in the first place.